The present invention relates to the field of floating offshore structures for the drilling and production of offshore deposits of petroleum and natural gas. In particular, it relates to a system for securing two floating structures together, one of which is a floating production facility kept in position by its own mooring/station-keeping system, and the other being a tender-assist drilling unit (TADU) partly moored to the seabed by at least four aft mooring lines, with the forward part thereof being coupled to the production unit by a set of lashing lines. The result is a coupled station-keeping system for the two floating structures.
In many deepwater drilling and production installations, a floating tender assist drilling unit (TADU) is tied to an adjacent offshore production facility or platform to assist in the drilling and production operations. The TADU can be any type of semi-submersible or barge hull form. Both the TADU and the platform are typically moored to the seabed, and they are lashed to each other so as to restrict relative movement between the two structures, thereby to facilitate the transfer of drilling consumables and personnel from one structure to the other and hook-up of control and fluid lines between the two structures. In the current state of the art, a semi-submersible TADU is moored with a minimum of eight mooring lines in addition to the mooring/station-keeping system of the production facility, which may be a Spar or tensions-leg platform (TLP). Current lashing systems are based on connecting the two floating structures with a pair of lashing lines to control the separation distance and the low and average frequency motions. In a typical coupling system, a number of mooring lines will cross, complicating the seabed layout. Furthermore, meeting critical performance criteria, as explained below, has proven difficult in practice.
In a practical TADU-to-production facility coupling or lashing system, the lashing mechanism must be capable of maintaining the relative movement within predefined limits that allow normal operation throughout environmental conditions that can be expected during the course of a normal year (a “one-year environment”), and that allow limited operations, including the maintenance of drilling circulation and control, throughout worst-case conditions to be expected during a typical ten-year period (a “ten-year environment”). In a 100-year extreme weather condition, the system must be capable of increasing the separation between the structures to a storm-safe distance, and at the same time function as 100-year storm-safe coupled mooring system.
The lashing system must also be capable of maintaining an optimum distance between the two structures during normal operations, while relative motions between them are absorbed. When necessary, the lashing system must allow distance between the two structures to be controllably increased, while maintaining the linkage between them. Thus, it is important, that the lashing system have a relatively low stiffness, so that there is relatively little coupling of the motions between TADU and the platform. Optimally, the lashing system will act as a pre-tensioned spring between the two structures, wherein the spring constant is such that the combination of maximum tension load and restriction in relative motion can assure drilling operability for a one-year environment, and at least a limited operability for a ten-year environment. Furthermore, the lashing system should minimize yaw.
To date, there is an unfulfilled need for a system for lashing together two floating structures that satisfactorily meets criteria set forth above.